Loop Cut Coordination technique
In the tutorial, the side 'rails' down which the edge is moving are parallel, so a translation (GY, for example) is equivalent to a slide (GG). On a triangle, that's not the case.

You can make a numerical entry for the distance of a translation, because all affected components move by the same amount. But you can't, by distance, for a slide, because vertices in a sliding loop will move different distances, depending on the location of adjacent vertices on the rails. 

Instead, a numerical entry for a slide represents the fraction of the distance to adjacent vertices, where 0 is the current position, and distances to adjacent vertices are mapped to  1 and -1.

You might be able to use that fraction, with a bit of mental arithmetic, but usually, for accuracy, I find myself G translating single vertices down edges using Custom Orientations derived from them where necessary, and as far as possible thinking like a carpenter.. finding ways to transfer measurements from one part of a model to another.